Choosing the right cable type isn’t just about picking a length and diameter – it involves considering a range of factors such as current capacity, voltage drop and installation conditions. Poor choices can lead to cables overheating, insulation breakdown, long term energy loss and losses in running costs.
Whether you’re an experienced electrician or a DIY enthusiast, understanding how to properly calculate cable size is essential for any electrical project. In this guide, we’ll walk you through the importance of cable sizing, the key factors to consider, and provide you with a step-by-step process to help you find the right cable size for your project.
What is cable size?
To a novice, cable size may just sound like the length and diameter of a cable. However, those electrically-minded will tell you that cable size actually refers to the physical and electrical characteristics (also usually referred to as a cross sectional area) of an electrical cable, which determine its suitability for specific applications.
Why is cable size important?
You may be wondering why cable size is important. Finding the proper cable size that suits the specific application of the cable ensures:
1. Safety
Accurate cable size helps to avoid electrical fires due to overheating and ensures proper cable insulation.
2. Efficiency
Proper cable size prevents energy loss due to resistance, which in turn will minimise a loss in running costs.
3. Longevity
Calculating the correct cable size prevents insulation breakdown over time and short circuits, overall minimising any maintenance costs.
4. Compliance
Proper selection ensures compliance with the UK safety regulations, such as BS 7671: The IET Wiring Regulations.
Key factors in cable sizing
Cable sizing refers to the physical and electrical properties that determine a cable’s suitability for a given application. This includes:
1. Load current
Load current is the electrical current (measured in amperes) that a cable must safely carry under normal operating conditions.
2. Voltage drop
Voltage drop is the reduction in electrical voltage that occurs as electric current flows through the cable due to its resistance. Voltage drop increases with longer distances and higher current loads. Excessive voltage drop can cause equipment to operate inefficiently or malfunction.
3. Cable length
Longer cable runs have higher resistance, and this leads to greater voltage drop. To compensate, a larger cable size (with a lower resistance per meter) may be required to meet voltage drop limits.
4. Ambient conditions
When calculating cable size, you need to consider the conditions of the cable in use, such as temperature, installation in insulation, burial, or exposure to direct sunlight – all of which can affect a cable’s ability to dissipate heat. Here, you may need to look into high temperature cables or harsh environment cables.
5. Conductor material
The conductor material, typically copper and aluminium, affects both conductivity and cost. Copper offers higher conductivity, allowing for smaller cable sizes, while aluminium requires larger cross-sectional areas for equivalent performance.
6. Installation method
The installation method, such as clipped direct, in conduit or buried, directly influences heat dissipation and current-carrying capacity. More restrictive installation methods may require increased cable sizes.
How to calculate a cable size
If you’re not an electrician and just want to figure out what size cable you need, here’s a simplified step-by-step guide to help you pick the perfect cable for you.
Step 1: Figure out the power of what you’re plugging in
First, look at the devices or appliances you’re using. Every device has a power label, usually in watts (W) or kilowatts (kW).
Example: a heater might say 2000 W, and a fridge might say 500 W.
Add up the total power of all the devices you’ll use at the same time.
Example: Heater (2000 W) + Fridge (500 W) = 2500 W.
If the power is in watts, divide it by the voltage in your house (usually 230V in the UK) to get the current in amps: Power (watts) / Voltage (volts) = Current (amps).
Example: 2500 W / 230 V = 10.87 amps. This then tells you the cable needs to handle around 11 amps.
Why it matters: the current determines the minimum cable size required to carry electricity safely.
Step 2: Measure the distance
Then, you need to measure how far the cable will go, in meters. If the cable goes back, like in some circuits, double the distance.
Example: From your circuit breaker to the appliance might be 30 meters. If the cable goes back, double it. So, 30 x 2 = 60 meters.
Step 3: Check voltage drop
As electricity travels through the cable, it “loses” a bit of power. This is called voltage drop. Long cables tend to lose more power, as do thin cables.
You don’t want your appliance to get less electricity than it needs. So, you’ll need to pick a cable that keeps this loss small, usually less than 5% of your voltage (e.g., 11.5 V for 230 V).
Step 4: Pick a cable material
Cables are usually made of copper or aluminium. Copper tends to be better and smaller, but more expensive, whereas aluminium tends to be cheaper but thicker. For most home setups, copper is the way to go.
Step 5: Use a cable size chart
You don’t need to calculate everything yourself. Electricians and cable manufacturers use charts or a BS 7671 cable size calculator that tell you the cable size (in mm²) based on:
- Current (amps): How much electricity your devices need.
- Distance (meters): Longer distances need bigger cables.
- Type of installation: Is the cable in the air, buried or in a wall?
Example: If your device needs 11 amps, and the cable runs 60 meters, you might need a 4mm² copper cable. If it’s a shorter distance, say 10 meters, a smaller 2.5mm² cable might work.
Step 6: Adjust for heat or other factors
If the cable is in a hot place (like an attic) or bundled with other cables, it gets hotter. Hot cables carry less electricity safely, so you may need a bigger size. Refer to the chart or calculator, or a professional, for advice.
Step 7: Double-check the rules
In the UK, cables must meet the electrical code BS 7671: The IET Wiring Regulations. This code tells you the minimum cable size to use for safety. Always make sure your choice meets the code.
Example in simple terms
- Device: 2000 W heater.
- Voltage: 230V.
- Current = 2000 / 230 = 8.7 amps
- Cable length: 50 meters one-way (100 meters total).
For 8.7 amps over 100 meters, you might need a 4mm² copper cable. If it’s a hot attic, you might choose a bigger 6mm² cable for safety.
Common mistakes to avoid when calculating cable size
Getting cable size right is essential for safety, efficiency and staying compliant with regulations. Below are some of the most common mistakes, along with why they matter:
1. Ignoring voltage drop
Not allowing for voltage drop, especially over longer cable runs, can cause problems at the point of use. Equipment may not receive enough voltage to operate properly, leading to poor performance or even failure. Motors, for example, may struggle to start or overheat if the voltage is too low.
2. Overlooking current-carrying capacity
Choosing a cable based only on its size in mm² without checking its current rating is a common issue. If the cable cannot safely carry the load, it can overheat, damage insulation and increase the risk of fire or failure.
3. Failing to consider installation conditions
Cable ratings are often taken from tables, but these assume ideal conditions. In reality, factors such as ambient temperature, grouping of cables or insulation can reduce how well a cable can dissipate heat. If these are not taken into account, the cable may not perform as expected.
4. Neglecting short-circuit performance
It’s important to check that a cable can withstand short-circuit currents for the required duration. If not, it could fail during a fault, leading to serious damage and safety risks.
5. Incorrect calculation of load demand
Underestimating the total load, or not allowing for future expansion, can result in cables that are too small and prone to overheating. On the other hand, oversizing cables unnecessarily can increase costs. A balanced approach is key.
6. Disregarding reactive loads
In systems with inductive or capacitive loads, ignoring power factor can lead to underestimating the actual current. This means the cable may need to be larger than initially expected to handle the true demand.
7. Using incorrect standards or codes
Not working to the correct standards, such as BS 7671, can result in installations that are unsafe or non-compliant, potentially leading to legal issues or costly remedial work.
8. Misjudging cable length
Underestimating the length of a cable run increases resistance and, in turn, voltage drop. If this is not properly accounted for, it can affect performance and push the installation outside acceptable limits.
9. Overlooking economic factors
Trying to save money upfront by choosing smaller cables can be a false economy. Higher resistance leads to greater energy losses over time, which can increase running costs and outweigh any initial savings.
Frequently asked questions about cable sizing
How do I account for future load expansion in cable sizing?
A simple approach is to build in some spare capacity – for example, designing the cable so that the current load uses around 70% to 80% of its capacity leaves room for future additions.
What is the difference between single-core and multi-core cables?
Single-core cables contain one conductor and are typically used for straightforward power distribution. Multi-core cables contain several conductors within one sheath and are more suited to complex systems such as control panels or equipment with multiple connections.
Do harmonics affect cable sizing?
Yes, they can. Harmonics from non-linear loads such as variable speed drives or LED lighting can increase the effective current in the cable. In some cases, this means a larger cable is needed to avoid overheating.
What happens if I oversize a cable?
Oversizing can increase material and installation costs and make cables harder to handle. While it can reduce voltage drop and energy losses, it’s not usually necessary unless you are planning for future expansion or have specific design requirements.
When it comes to cables and cable manufacturing, we’re the experts. To learn more about what type of cable you need, explore more resources like this available on our blog, or contact our helpful team today for tailored advice and support.